Explain the structure of a router.
Answers
Routing Architecture
A network router is a network device with interfaces multiple networks whose task is to copy packets from one network to another based on the routing tables stored in the memory. Router will typically utilize one or more routing protocols, such as RIP, OSPF, or BGP. Routers also accept routes which are configured manually by a network administrator. Those routes are called static routes. The router will use this information to create a routing table. The network router will then use its routing table to make intelligent decisions about which packets to copy to which of its interfaces. This process is known as routing.
Router has four components: input ports, output ports, the routing processor and the switching fabric.
a) Input port performs the physical and data link layer functions of the router. The bits are constructed from the received signal, packet is de-capsulated from the frame, errors are detected and corrected. The packet is ready to be forwarded by the network layer. Input ports has buffers (queues) to hold the packets before they are directed to the switching fabric.
b) An output port performs the same function as the input port, but in the reverse order.
c) Routing Processor: The destination address is used to find the address of the next hop. Routing processor searches routing tables.
d) Switching Fabrics: It moves the packet from the input queue to the output queue. In the past, memory of the computer or a bus was used as the switching fabric. The simplest type of switching fabric is the crossbar switch which connects n inputs to n outputs in a grid, using electronics micro-switches at each cross point.
The functionality of the router is as follows:
1) Route Processing: This includes routing table construction and maintenance using routing protocols such as RIP or OSPF etc. The routing tables could be created statically or updated dynamically.
2) Packet forwarding: The router checks the received packet (checking header field, version number etc) and calculates the header checksum. The router also performs a table lookup to determine the output port to direct the received packet and the next hop to which to send the packet along the route. This is based on destination IP address and the subnet mask(s) of the associated table entries. It could be local delivery, unicast delivery or could also be multicast delivery.
3) Packet lifetime control: The router adjusts the TTL field in the packet header which is used to prevent packets from circulating endlessly throughout the internetwork. TTL value is decremented and packet will be delivered to the associated output port.
4) Checksum calculation: IP header checksum must be recalculated due to the change in the TTL field. Sometimes IP packets must also be fragmented based on the MTU (Maximum Transmission Unit) field.
5) Special services: Packet translation, encapsulation, traffic prioritization, authentication and access services such as packet filtering for security/firewall purposes are also includes under this category.